BRIGHTNESS DISCRIMINATION AS A FUNCTION OF THE DURATION OF THE INCREMENT IN INTENSITY.

1. This investigation has been concerned with an analysis of brightness discrimination as it is influenced by the duration of DeltaI. The durations used extend from 0.002 second to 0.5 second. 2. DeltaI/I values at constant intensity are highest for the shortest duration and decrease with an increase in duration up to the limits of a critical exposure time. At durations longer than the critical duration the ratio DeltaI/I remains constant. 3. The Bunsen-Roscoe law holds for the photolysis due to DeltaI. This is shown by the fact that, within the limits of a critical duration, the product of DeltaI and exposure time is constant for any value of prevailing intensity, I. 4. At durations greater than the critical duration the Bunsen-Roscoe law is superseded by the relation DeltaI = Constant. This change of relation is considered in the light of Hartline's discussion (1934). 5. The critical duration is a function of intensity. As intensity increases the critical duration decreases. 6. Hecht's theory (1935) accounts for the data of this experiment if it be assumed that brightness discrimination is determined by a constant amount of photolysis.